One speaker I have been a fan of and particularly for its bass performance is the Linn Isobarik.

What I am planning is a stereo speaker system using a high quality Seas Full range unit in a small module mounted onto of a woofer enclosure which will be mounted 4 B139 bass units in an Isobarik configuration.

Series parallel connection this will realise a relatively kind 8 ohm load. Two woofers will be mounted on the front baffle with two units closely coupled behind them so that these inner woofers then see the inside of the main enclosure.

Now the crossover initially will be passive, a simple series inductor with parallel resistance to tune the upper midrange of the Seas FR unit, then a straightforward third order network on the Bass.

Likely crossover point will be around 300 Hz or so with a 1st order on the Seas and the Bass network probably from the CS7 kit to begin with.

Now to model the cabinet for the Isobarik system.

I will tune low so that system Q is around or below 0.5 after the crossover has added its additional resistance to the drivers.